Storing a long length of optical fiber on a supporting former raises the problem of differential expansion between the supporting former and the coil of optical fiber. The coil has a coefficient of axial expansion (expansion along the axis of the supporting former) that is much higher than its coefficient of radial expansion (in the direction of increasing circumferential length of the turns). An optical fiber is made up of a silica portion comprising the core and the cladding of the fiber, which portion has a coefficient of expansion .alpha..sub.1 =5.times.10.sup.-6 and is protected by a covering having a coefficient of expansion .alpha..sub.2 = in the range 100.times.10.sup.-6 to 900.times.10.sup.-6 depending on temperature.
Thus, in the axial direction of the fiber, the much higher modulus of the silica restrains the expansion of the covering and limits the longitudinal expansion thereof to the longitudinal expansion of silica, i.e. to about 5.times.10.sup.-6.
In the radial direction of the fiber, the expansion of the silica and of the covering in combination gives an overall coefficient approximately in the range 50.times.10.sup.-6 to 450.times.10.sup.-6.
Thus, once the fiber is wound on the supporting former, the coefficient of axial expansion of the coil is in the range 50.times.10.sup.-6 to 450.times.10.sup.-6, while its coefficient of radial expansion is 5.times.10.sup.-6. These coefficients of expansion are to be compared, for example, with the coefficient of expansion of a supporting former made of aluminum. For such a supporting former, the coefficient of expansion is 24.times.10.sup.-6 both axially and radially.
During temperature variations, the stresses induced by these various values for the coefficients of thermal expansion are sufficient to displace the turns of the coil relative to one another, and to destroy the coil which can then no longer be paid out rapidly without breaking the fiber.
European Document No. 0 747 741 describes a reel of optical fiber in which the problem is solved by means of a thermal expansion matching layer situated between the supporting former and the bundle constituting the coil of optical fiber. That layer is made of an elastomer material, such as silicone, for example, or else it is made of a composite material made of a soft elastomer matrix into which hard and rigid fibers are inserted, such as glass fibers or carbon fibers oriented in the circumferential direction of the supporting former.
Unfortunately, that matching layer, which is in direct contact with the optical fiber coil, can cause stresses in the coil during temperature variations.